1. FIELD OF THE INVENTION
The present invention relates to a sensor signal processing apparatus for processing an output signal from a sensor for detecting physical information for the purpose of display or the like.
2. DESCRIPTION OF THE PRIOR ART
A recently commercially available electronic timepiece has, in addition to normal functions such as a time display function, an alarm function, and a chrono function, a so-called sensor function for measuring constantly-changing physical information such as a pressure, e.g., an atmospheric pressure or a water pressure or a temperature by a sensor and displaying the information through a signal processor. However, in the electronic timepiece, a space for housing a circuit element is limited, and a small battery is used as a power source. In addition, a service life as a timepiece must be assured for at least two years. Therefore, the sensor function which also uses the battery as a power source must satisfy the following conditions. First, a power consumption required for measuring and displaying the physical information must be reduced as low as possible so as to prolong a battery life. Second, in order to assure reliability of the displayed physical information, an alarm must be reliably generated when a battery voltage is reduced below a predetermined value for maintaining measurement, thereby indicating necessity for replacement of a battery. Third, an element for adjustment such as a variable resistor is omitted as much as possible, and adjustment is eliminated as much as possible. A conventional technique concerning the above conditions will be described below.
In the above battery-driven electronic timepiece, a sensor is used to detect constantly-changing physical information such as a pressure or temperature, and a physical information signal output from the sensor is processed by a signal processor and then supplied to a rear-stage circuit such as a display. When the display is connected to the signal processor as a rear-stage circuit, the signal processor adjusts correspondence between the physical information detected by the sensor and a display value. A conventional adjustment in an atmospheric pressure display unit will be exemplified below.
FIG. 1 shows a conventional sensor signal processing apparatus used in a battery-driven electronic timepiece.
In FIG. 1, reference numeral 1 denotes an atmospheric pressure sensor for outputting an atmospheric pressure signal S.sub.1 proportional to an atmospheric pressure P; 2, a sensor driver for supplying a constant current to the atmospheric pressure sensor 1 to drive it; and 3, an amplifier for amplifying the atmospheric pressure signal S.sub.1. The amplifier 3 consists of a sensitivity adjusting resistor 3a and an offset adjusting resistor 3b for adjusting an amplification factor and an offset, respectively. Reference numeral 4 denotes an A/D converter for A/D-converting an amplified pressure signal S.sub.1 ' output from the amplifier 3 and outputting the signal as converted data D.sub.c ; 5, a sensor information data processor, constituted by a microcomputer, for converting the converted data D.sub.c into sensor information data D.sub.j and outputting it; and 6, a display unit for displaying an atmospheric pressure value in accordance with the sensor information data D.sub.j.
An operation of the sensor signal processing apparatus having the above arrangement will be described below.
When the atmospheric pressure sensor 1 is driven by a constant current supplied from the sensor driver 2, it outputs the atmospheric pressure signal S.sub.1 proportional to the atmospheric pressure P applied on it. The pressure signal S.sub.1 is amplified by the amplifier 3 to be the signal S.sub.1 ', and the signal S.sub.1 ' is converted into the converted data D.sub.c by the A/D converter 4. The sensor information data processor 5 has the following predetermined characteristic equation (1): EQU D.sub.j =a.times.D.sub.c +b (1) [a, b: constants]
The sensor information data processor 5 receives the converted data D.sub.c and calculates the sensor information data D.sub.j in accordance with sensor characteristic equation (1). The calculated sensor information data D.sub.j is displayed on the display unit 6.
The sensor information data D.sub.j represents the atmospheric pressure P applied on the sensor 1, and the atmospheric pressure P is displayed on the display unit 6. However, since the sensor characteristic equation (1) of the sensor information data processor 5 is fixed, correspondence cannot often be obtained between the atmospheric pressure P and the sensor information data D.sub.j due to variations in sensitivity and offset of the atmospheric pressure sensor 1. Therefore, the sensitivity adjusting resistor 3a and the offset adjusting resistor 3b of the amplifier 3 must be adjusted in advance.
This adjustment is performed as follows.
First, a given atmospheric pressure P.sub.1 is applied on the atmospheric pressure sensor 1, and the sensitivity and offset adjusting resistors 3a and 3b are adjusted so that a value P.sub.1 is displayed on the display unit 6. Then, an atmospheric pressure P.sub.2 different from the atmospheric pressure P.sub.1 is applied on the atmospheric pressure sensor 1, and the sensitivity and offset adjusting resistors 3a and 3b are adjusted so that a value P.sub.2 is displayed on the display unit 6. In this case, however, when adjustment is performed at the atmospheric pressure P.sub.2, adjustment performed at the atmospheric pressure P.sub.1 is deviated. For this reason, adjustment is repeatedly performed at the atmospheric pressures P.sub.1 and P.sub.2 until both the values P.sub.1 and P.sub.2 are correctly displayed on the display unit 6.
When both the values P.sub.1 and P.sub.2 are correctly displayed on the display unit 6, the atmospheric pressure sensor 1 can output the atmospheric pressure signal S.sub.1 proportional to the atmospheric pressure because it is a linear sensor. Therefore, a correct pressure can be displayed on the display unit 6 throughout a measurement range.
As described above, in the conventional sensor signal processing apparatus, the sensitivity adjusting resistor 3a and the offset adjusting resistor 3b must be provided in the amplifier 3 and adjusted in the above manner. In order to perform the above adjustment, the atmospheric pressures P.sub.1 and P.sub.2 must be alternately applied a large number of times, resulting in a time-consuming operation. In addition, the adjustment operation must be manually performed. Furthermore, the sensitivity and offset adjusting resistors 3a and 3b are mechanically adjusted and hence are very unstable. Therefore, even if adjustment is correctly performed in the above manner, the sensitivity and offset may be deviated from the corrected values by vibrations or may change over time.
FIG. 2 shows a circuit for detecting a battery voltage of a power source unit of the conventional sensor signal processing apparatus. Such a battery voltage detector is disclosed in, e.g., U.S. Pat. No. 4,043,110.
In FIG. 2, reference numeral 7a in a voltage detector 7 denotes a P-channel MOS transistor (to be referred to as a P-MOS hereinafter). The source and the bulk of the P-MOS 7a are connected to a battery voltage V.sub.dd, its gate is connected to a reference potential V.sub.ss, and its drain is connected to the reference potential V.sub.ss through a resistor R.sub.v Reference numeral 7b denotes a P-MOS; and 7c, an N-channel MOS transistor (to be referred to as an N-MOS hereinafter). The source and the bulk of the P-MOS 7b are connected to the battery voltage V.sub.dd, and those of the N-MOS 7c are connected to the reference potential V.sub.ss.
The gates of the P-MOS 7b and the N-MOS 7c are connected to a node between the drain of the P-MOS 7a and the resistor R.sub.v, and their drains are connected with each other to form an output terminal 0. That is, the P-MOS 7b and the N-MOS 7c constitute an inverter.
The conventional voltage detector 7 having the above arrangement utilizes nonlinearity of the P-MOS 7a, linearity of the resistor R.sub.v, and the fact that a logical threshold voltage of the inverter constituted by the P-MOS 7b and the N-MOS 7c is about 1/2 of the battery voltage V.sub.dd. In this case, since the characteristics of the P-MOS 7a vary due to manufacturing limitations, a resistance of the resistor R.sub.v is adjusted to detect that the battery voltage V.sub.dd is reduced to a desired voltage.
When an IC arrangement is adopted in the voltage detector having the above circuit arrangement, variations are increased. Therefore, adjusting resistors must be provided to perform adjustment, resulting in a time-consuming, costly operation.
In addition, the voltage set by the above adjustment is a constant value which is determined by adding a margin to the lowest voltage at which an IC can operate in consideration of variations in ICs. Therefore, in some ICs, a battery is replaced with a new one although its voltage is sufficient for an operation.